It
must be said that some of my fellow Calvinists aren’t that happy about
this verse being used in evangelism. The context (which I readily acknowledged in the sermon) relates to a
backslidden church (Laodicea) and they say that these words are
inappropriate to use to sinners who are dead in trespasses and sins. Indeed, one Reformed blogger uses language like this:

"In fact, dare I say it, but that the application of this verse to
evangelism actually demeans Christ. It reduces our sovereign Lord to be
the helpless and often rejected beggar always so "meekly" knocking on
everyone's doors, and most of them will reject Him anyway. It dethrones
God and elevates Man, as if Man is the center of all things. Such an
Arminian methodology compromises the person of Christ and the Godhead,
and therefore dishonors the Lord we claim to worship."

A quick look at some classic Calvinist commentaries and sermons, however, will show that even the most Reformed of men never hesitated, at least, to apply it to sinners in the gospel. The honoured list of commentators includes Matthew Henry, Matthew Poole, Jamieson, Fausett and Brown, Albert Barnes and William Hendriksen. Among the Reformed preachers, Robert Murray McCheyne preached it in the gospel.

I am unaware if John Calvin ever commented on the verse at hand or preached it. What I do know, however, is this: He made an unreferenced reference (fancy that!) to it in his comments on John 8:21. As I give the quote, note the context (and indeed reference to) stubborn sinners going to Hell. Emphasis mine:

"John 21. I go. Perceiving that he is doing no good among these obstinate men, he threatens their destruction; and this is the end of all those who reject the Gospel. For it is not thrown uselessly into the air, but must breathe the odour either of life or of death, (2 Corinthians 2:16.) The meaning of these words amounts to this. “The wicked will at length feel how great loss they have suffered by rejecting Christ, when he freely offers himself to them. They will feel it, but it will be too late, for there will be no more room for repentance.” And to alarm them still more by showing them that their judgment is near at hand, in the first place, he says that he will soon go away, by which he means that the Gospel is preached to them only for a short time, and that if they allow this opportunity to pass away, the accepted time and the days appointed for salvation (Isaiah 49:8; 2 Corinthians 6:2) will not always last. Thus also, in the present day, when Christ knocks at our door, we ought to go immediately to meet him, lest he be wearied by our slothfulness and withdraw from us. And indeed we have learned, by many experiments in all ages, how greatly this departure of Christ is to be dreaded.

And you shall seek me. We must first ascertain in what manner the persons now spoken of sought Christ; for if they had been truly converted, they would not have sought him in vain; because he has not falsely promised that, as soon as a sinner groans, he will be ready to assist him. Christ does not mean, therefore, that they sought him by the right way of faith, but that they sought him, as men, overwhelmed by the extremity of anguish, look for deliverance on every hand. For unbelievers would desire to have God reconciled to them, but yet they do not cease to fly from him. God calls them; the approach consists in faith and repentance; but they oppose God by hardness of heart, and, overwhelmed with despair, they exclaim against him. In short, they are so far from desiring to enjoy the favor of God, that they do not give him permission to assist them, unless he deny himself, which he will never do. "

Hmmm! Fair enough, Calvin does not actually use the phrase "heart's door" but it is clearly implied, especially when Calvin talks about those who "oppose God by hardness of heart."

William Hendreksen in his "More than Conquerors" comment on this verse rightly points out that Revelation 3:20 emphasises the doctrine of man's conversion where he is active. In regeneration, man (as we know) is passive because regeneration is the sovereign work of God. However, it results in man's conversion where it is man who responds to the gospel and man exercises his God given faith and repentance. (God does not respond for the sinner, but enables him to respond.) In Acts 16:14 where we read that it was the Lord who opened Lydia's heart, the viewpoint there is that of regeneration, resulting (cause and effect) in her attending to the things that were spoken of Paul and therefore her conversion.

So I had not the least qualm (this blog post notwithstanding) in preaching Revelation 3:20 last night in the gospel and aimed my exhortations to "open your heart's door" towards the unsaved gathered in the meeting.